34 



PHYSICAL GEOLOGY 



Summary. The more important points concerning ig- 

 neous rocks may be summarized as follows : (1) Igneous rocks 

 are formed by the solidification of lavas. (2) Although they 

 contain many minerals, a few minerals make up the great 

 mass of the igneous rocks. The most important are (a) 

 quartz, (6) feldspar, (c) the ferromagnesian minerals, and (d) 

 the iron oxides. (3) Chemically, igneous rocks may be divided 

 into three great classes acidic, neutral, and basic. (4) The 

 physical character of igneous rocks is determined by (a) the 

 character of the parent lava, and (6) the conditions under 

 which it solidified. (5) Since both the composition of lavas 

 and the circumstances attending their solidification vary 

 widely, many kinds of igneous rocks result. Of these the few 

 that have been mentioned are most important. They are 

 classified in the accompanying table. 



CLASSIFICATION OF IGNEOUS ROCKS 1 

 A. GRAINED, CONSTITUENT GRAINS RECOGNIZABLE. MOSTLY INTRUSIVE 



B. DENSE, CONSTITUENTS PARTLY OR WHOLLY UNRECOGNIZABLE. INTRUSIVE AND 



EXTRUSIVE 



C. ROCKS COMPOSED WHOLLY OR IN PART OF GLASS. EXTRUSIVE 



Nonporphyritic 



OBSIDIAN, PITCHSTONE, PUMICE, ETC. 



Porphyritic 



OBSIDIAN-PORPHYRY, PITCHSTONE-POHPHYRY 



D. FRAGMENTAL IGNEOUS MATERIAL. EXTRUSIVE 



TUFF, VOLCANIC BRECCIA 



1 After Pirsson, with slight modification. 



